Efficient electrocatalytic oxygen reduction reaction of thermally optimized carbon black supported zeolitic imidazolate framework nanocrystals under low-temperature

Turning commercially available low-cost conducting carbon black materials into functional electrocatalytic electrode media using simple surface chemical modification is a highly attractive approach. This study reports on remarkably enhanced oxygen electrocatalytic activity of commercially available Ketjenblack (KB) by growing a non-precious cobalt metal-based zeolitic-imidazolate framework (ZIF-67) at room temperature in methanol solution followed by a mild thermolysis. The resulting Co@CoOx nanoparticle decorated nitrogen-doped KB derived from the optimized ZIF-67 : KB weight ratio of hybrid samples at 500–600 °C shows high performance for the oxygen reduction reaction (ORR) with impressive Eonset and E1/2 values of ∼0.90 and ∼0.83 V (vs. RHE), respectively in 0.1 M KOH electrolyte. Such ORR activity is comparable to, or better than many metal@metal-oxide-carbon based electrocatalysts synthesized under elevated carbothermal temperatures and using multicomponent/multistep chemical modification conditions. Therefore, a simple electrocatalyst design reported in this work is an efficient synthesis route that not only utilises earth-abundant carbon black but also comprises scalable room temperature synthesized ZIF-67 following mild thermolysis conditions under 600 °C.

The weight percentage of ZIF-67 grown on the surface of KB was calculated by comparing the final product weight (W1) of ZKB and the original KB weight (W2), the weight percentage (wt%) of ZIF-67 in the sample = (W1 -W2) x 100/W1.
The other composition ZKB samples, with about 60, 40 and 20 wt% of ZIF-67 in ZKB (named 60ZKB, 40ZKB and 20ZKB), were synthesized using the same procedure as above, by changing the quantity of precursors and solvent used as listed in the Table S1 below.
Electronic Supplementary Material (ESI) for RSC Advances.This journal is © The Royal Society of Chemistry 2023 Thermolysis of ZIF-67, and 80ZKB, 60ZKB, 40ZKB and 20ZKB samples Typically, 30-50 mg of ZIF-67 and/or ZKB samples each in alumina boats were placed into a horizontal tube furnace (Zhengzhou TCH instrument Co., Ltd.), which was Ar purged at room temperature, and thermolysis of the samples was carried out for 6 hours at a given temperature of 500 or 600 or 700 °C under a continuous flow of argon gas at a heating rate of 5 per minute.The thermolyzed ZIF-67 samples at 500, 600 and 700 °C were named as ZIF-500, 600 and ZIF-700, respectively.Likewise, thermolyzed 80ZKB samples at 500, 600 and 700 °C were named as 80ZKB-500, 80ZKB-600 and 80ZKB-700, respectively.The same notation applied for thermolyzed 60ZKB, 40ZKB and 20ZKB samples.

Characterization
Powder XRD (Co Kα radiation, Panalytical) was carried out in the scan range of 2θ = (2-80)° and step size of 0.013°.X-ray photoelectron spectroscopy (XPS, AXI, Supra, Kratos) data, scanning electron microscopy (SEM, Aztec Live ULTIM) and transmission electron microscopy (TEM, JEM-2100Plus) measurements were carried out on the samples supported on a carbon tape or a carbon-coated copper TEM grid.

Electrochemical measurements
The electrochemical measurements were conducted using a Bio-Logic electrochemical workstation (DHS Instruments Company) with a standard three-electrode system in O 2 -purged 0.1 M KOH electrolyte.Ag/AgCl (in a saturated KCl aqueous solution) and Pt-wire served as reference and counter electrodes, respectively.The working electrode was a glassy carbon-based rotating disk electrode (RDE, Ф = 5 mm, area = 0.19625 cm 2 ).For the electrochemical tests, the catalyst was prepared as follows: 5.00 mg powder sample was dispersed in 1 mL Nafion/deionized water (40 μL/960 μL) solution via sonicate for about 30 to 45 minutes to form a homogeneous ink.Then, 10.0 μL as-prepared catalyst ink was pipetted and drop-casted onto the polished clean and dry RDE and allowed to dry at 45 °C for about half an hour.Catalyst loading per unit electrode area of all the samples was about 0.254 mg cm −2 .The higher catalyst loadings of 0.318 and 0.382 mg cm −2 were obtained by increasing the drop-casting ink volume to 12.5 and 15.0 μL, respectively.Cyclic voltammetry (CV) curves were measured at a scan rate of 50 mV s -1 initially until stable overlapping CV curves were obtained (which requires 20 to 50 scans) and actual data was recorded at a scan rate of 10 mV s -1 in the potential range of +0.2 V to -0.8 V. Linear sweep voltammetry (LSV) polarizations were recorded at a scan rate of 10 mV s -1 in the same potential range of +0.2 V to -0.8 V. ORR LSV curves were measured at rotational speed of 1600 rpm.The measured potentials (vs.Ag/AgCl) were converted to be relative to the reversible hydrogen electrode (RHE) using the equation: V vs. (RHE) = V vs. (Ag/AgCl) + 0.059 × pH +0.197.Figure S3.Comparative LSV curves of 20ZKB samples thermolyzed at 500, 600 and 700 °C; Among these, 20ZKB-600 shows better ORR performance.

Figure S2 .
Figure S2.Comparative LSV curves of 40ZKB and 60ZKB samples thermolyzed at a) 500 °C and b) 700 °C.This shows better ORR performance of thermolyzed 60ZKB samples over thermolyzed 40ZKB samples.

Figure S4 .
Figure S4.LSV curves of 60ZKB-600 and Pt/C samples measured with higher catalyst loadings of 0.318 mg cm −2 (noted as 1.25 x mass) and 0.382 mg cm −2 (noted as 1.50 x mass), which were obtained by increasing drop-casting catalyst ink volume to 12.5 and 15.0 μL, respectively, compared to 0.254 mg cm −2 loading from 10.0 μL volume of catalyst ink in all other tested samples.This data shows an improved limiting current density compared to the data showed in Figures 2 and Figures S2-S3.

Table S1 .
Precursor and solution amounts for synthesis of 80ZKB, 60ZKB, 40ZKB and 20ZKB samples

Table S2 .
Literature reported M@MO x -N-C based catalysts with carbonization temperature and ORR performance E onset , E 1/2 and J L values in a 0.1 M KOH electrolyte.